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In this work, the novel reaction mechanism initiated by the fast CH( 2 Π) + C 2 H 2 f C 3 H 2 + H reaction (r13a) and followed by C 3 H 2 + O f C 2 H + HCO (or H + CO) (r25a) was established as the dominant C 2 H formation pathway in low-pressure acetylene/atomic oxygen flames at 600 K. The C 2 H 2 /O/H flames were investigated in an isothermal discharge-flow reactor at a pressure of 2 Torr, with He as bath gas. Concentration vs reactiontime data were obtained by molecular beam sampling and threshold ionization mass spectrometry. The crucial role of CH( 2 Π) was evidenced by CH 4 -addition experiments on room-temperature C 2 H 2 /O/H systems, where CH( 2 Π) and CH 2 ( 1 A 1 ) are the sole intermediates that react rapidly with CH 4 . The observed strong reduction of [C 3 H 2 ], [C 2 H], and [C 4 H 2 ] upon CH 4 addition could be correlated quantitatively with the known removal of CH( 2 Π) by CH 4 . The reaction channels r13a and r25a as sources of C 3 H 2 and C 2 H, respectively, were each established by quasi-steady-state analyses of the pertaining radicals in C 2 H 2 /O/H mixtures at 600 K. By a similar method, the observed C 3 H radicals could be attributed to a minor channel of the CH( 2 Π) + C 2 H 2 reaction (r13) parallel to that producing C 3 H 2 . At 600 K and 2 Torr, the following approximate product yields of reaction r13 were derived: 85 -19 +9 % C 3 H 2 plus H, and 15 -9 +19 % C 3 H plus H 2 . Concomitantly with the identification of reaction r25a as dominant C 2 H source, the rate constant of the reaction C 2 H + O (r19) was determined relative to the well-known kinetic coefficients of C 2 H + C 2 H 2 and C 2 H + O 2 : k 19 ) (9 ( 4) × 10 -11 cm 3 molecule -1 s -1 at 600 K. It is suggested that a sizeable fraction of the ethynyl radicals formed in fuel-rich hydrocarbon flames is produced likewise by oxidation of C 3 H x radicals (x ) 1-3) that arise in the fast reactions of CH(X 2 Π) and CH 2 (a 1 A 1 ) with C 2 H 2 .

The CH2(g3Bl) + H -CH(X211) + H2 reaction (1) was investigated in CH2CO/O systems (7' = 400, 500, 650, and 950 K, p = 2 Torr) as well as in C2H2/0 systems (T = 590 and 890 K, p = 4 Torr) using dischargeflow/molecular beam sampling mass spectrometry techniques (D-FMBMS). The fiist rate coefficient data at temperatures intermediate between room and flame temperature are presented. The CH2(g3Bl) + H rate constant was measured relative to the well-known k(CH2(X3B1)the change of the quasi-steady state CH2 concentration upon varying the [H]/[O] ratio at given CH2 formation rate. In the temperature range of interest, the kl coefficient was found to exhibit a clear-cut negative temperature dependence. The following kl values were obtained (i) using the CH2CO + 0 reaction as CH2 source, (2.93 f 0.80) x at 650 K, and (1.18 f 0.32) x 1O-Io at 950 K; (ii) using the C2H2 -t 0 system as CHZ source, (1.90 f 0.58) x at 590 K and (1.12 f 0.36) x at 890 K (k in cm3 molecule-' s-I; 95% confidence intervals, includingpossible systematic errors). The data obtained with the two CH2 sources are in good mutual agreement. The decrease of kl with temperature is in accord with literature kl data at room temperature on one side and in the 1500-2500 K range on the other. However, the observed temperature dependence in the 300-1000 K range is much less steep than predicted by the recommended kl(T) expression in a recent evaluation.